Emission and reflection nebulae differ primarily in their interactions with light.
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Emission Nebulae: These nebulae emit their own light, primarily due to the ionization of gas (mostly hydrogen) by the ultraviolet radiation from nearby young, hot stars. They appear bright and colorful.
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Reflection Nebulae: These do not emit their own light but instead reflect the light of nearby stars. They typically appear blue because blue light is scattered more than other colors.
So, the main difference lies in their interactions with light: emission nebulae produce light, while reflection nebulae reflect light from stars.